Japan is a country that places a great emphasis on hygiene – so great that non-Japanese might find it either highly admirable or deathly paranoiac. Along with politeness, hygiene is one of the more important and prominent aspects of Japanese culture. And like politeness, the lengths Japanese people go to in order to maintain a high level of hygiene standards as part of their culture can be eye-popping.
The emphasis on good hygiene is inculcated in the Japanese from a very young age. At school, Japanese kids are constantly drilled about the importance of proper hygiene through various rules and lectures. For instance, spraying their hands with alcohol before lunch is mandatory. Though they may not actually care about it at their age, this doctrine is etched firmly in their minds as they reach adulthood.
An important part of Japanese hygiene culture is the practice of wearing a face mask. When you arrive in Japan, you'll more often than not see people wearing a mask walking around in public, inside buildings, out in the open... everywhere.
Japanese people usually wear a face mask for one of two reasons. Healthy people wear it to protect themselves from airborne diseases, such as a common cold. Sick people do so to prevent it from spreading to others, and in fact, they are expected to. A sick person who doesn’t wear a mask is frowned upon, because he or she could spread the sickness to others, thereby causing a nuisance to society.
In typical Japanese marketing strategy, face masks may come in various designs, from simple, thin white masks, to somewhat more expensive ones that are thicker, to those particularly aimed towards young girls, like Hello Kitty and Rilakkuma themed masks.
Wearing a face mask, regardless of what design it has, especially for an extended period of time, makes the wearer feel uncomfortable, either due to the feel of the material on his or her face, or the difficulty in breathing and talking, or both. This applies to the Japanese people as well. They do not like wearing a face mask, for whatever reason. Yet they do so, because it's part of being Japanese.
At school, it is very common to see teachers and students wearing a mask all day long if there's a cold or a flu sneaking around. Imagine a teacher conducting a lesson to a classroom of forty students, all of them wearing a mask (both teacher and students). Yup, this situation is entire possible, and is definitely not a rarity. And no, they actually do not like it.
So when you arrive in Japan, don't get bothered by the sight of menacing-looking people wearing a face mask. This is merely part of their hygiene culture. And since the only prominent part of their faces are their eyes, it is difficult to tell their true emotions... Welcome to Japan!